The Ink for a water-base ballpoint pen has a viscosity of as low as 50 mPa.multidot.s to 3 Pa.multidot.s, while the ink for a oil-base ballpoint pen, though is has a similar structure to a water-base one, has a viscosity of 3 to 20 Pa.multidot.s. Consequently, the ink filled in a water-base ballpoint pen may leak out when the pen is left upward or sideways. Moreover, even a small impact made on the pen may cause its ink to scatter and to stain the hand or the clothes. Therefore, the water-base ballpoint pen is equipped with ink follower for preventing such accidents.
There have been conventional arts for a water-base ballpoint pen with its ink in its ink reservoir that it is equipped ink follower composed of a gelled material, or a mixture of the gelled material and solid material. The aims of the arts are to make the ink follower follow the ink smoothly, to make the pen endure the impact when dropped, to prevent the ink from back flow, to give the pen a good appearance, and so on. A common feature of such arts is that the ink follower, which has pseudo-plasticity, is made from hardly-volatile or non-volatile solvent which is supplemented with some kind of thickener in order that the ink follower may not flow backward when the pen is left sideways or upward.
Another feature of such arts is that the known ink follower for water-base ballpoint pens often has very low viscosity and consistency, as compared with that for conventional oil-base ballpoint pens, which often has equal viscosity and consistency to common grease used for lubricant.
About as much as 50 to 300 mg of ink is required for writing a line of 100 m in length by a water-base ballpoint pen holding the ink in the ink reservoir, while only 10 to 30 mg of ink is required by an oil-base ballpoint pen.
Thus, the ink follower for the water-base ballpoint pens is required a strict ink-following performance, and is, therefore, mainly of low viscosity and consistency.
The ink follower for the water-base ballpoint pens consists of materials similar to the lubricant grease, and exhibits time-dependent behaviors based on similar physical laws.
Lubricant grease with low viscosity and consistency generally has such low stability that oily matter likely separates when left to stand. If the oily matter separates from ink follower, it affects writing adversely by reacting with surfactant in the ink, or by forming oil drops which block the ink passage.
The high mobility of thickener in the lubricant grease is likely to cause the grease to lose homogeneity easily by forming a mixture of coarse and dense portions. Ink follower lacking homogeneity is separated into a portion following ink and portions adhering to the inner wall of the ink reservoir. The adhering portions not only give the pen an unpleasant appearance, but also mean a corresponding loss of the ink follower, resulting eventually in its failure to function of, for example, preventing the ink from volatilizing or from leaking.
The lower the viscosity of the thickener of the grease is, the less effectively the thickener is dispersed by a disperser such as a two-roll mill, a three-roll mill, a kneader or a planetary mixer, any one of which is suitable for substances with high viscosity. The thickener is, however, not so low in viscosity as to be capable of being mixed effectively by a disperser such as a bead mill, a sand mill or a homogenizer, any one of which is suitable for substances with low viscosity. Inefficient dispersion causes not only time-dependent instability but also lot-to-lot instability in viscosity and uniformity.
The lubricant grease and the known ink follower have a common defect, too.
Namely, when they are used as ink follower in a water-base ballpoint pen which holds the ink in a cylindrical or similarly shaped ink reservoir with an inside diameter of 2.5 mm or larger, bubbles often occur between the ink and the ink follower by the passage of time. Moreover, bubbles or cracks, which have not seen initially, often occur in the ink follower (or the lubricant grease used as a substitute therefor) Namely, the greasy matter crack obviously. We, the inventors of this invention, call these phenomena "bubbling". Once the bubbling occurs between the ink and the ink follower, it grows larger and interrupts the contact between the ink and the ink follower. Then, the ink follower is urged by the vapor pressure of the ink toward the tail end of the pen, and eventually falls off. The ink follower having cracks or the like loses its function of keeping the ink from contact with the air.
These phenomena are presumably due to the invisibly fine bubbles that may exist in the ink follower or lubricant grease when manufactured. The bubbles gather with the passage of time, and tend to escape from the pen.
The bubbling is a serious defect in this kind of water-base ballpoint pen.
Commercially available ballpoint pens are subjected to a strong centrifugal force for debubbling. Debubbling by a strong centrifugal force is, however, not always effective for removing invisibly fine bubbles, but can only reduce the percentage of bubbling to about 1/5 to 1/20.
Moreover, the centrifuging is not a suitable method for debubbling for pens with a pigment ink, particularly the ink containing a pigment with a true specific gravity of 4 or higher, since a strong centrifugal force promotes the sedimentation of the pigment.
Fine bubbles can also be removed from ink follower when it is subjected to a reduced pressure. But the base oil for the ink follower is so high in viscosity that the bubbles which have expanded at a reduced pressure are not easily broken. Therefore, the method has a defect that the possible amount of the ink follower is limited to one-third to one-fifth of the capacity of a depressurizing vessel.
Considering above problems, the object of this invention is to dissolve the defect that conventional ink follower for a water-base ballpoint pen has lot-to-lot and time-dependent instability of quality, and to provide a method for manufacturing ink follower which has time-dependent stable performance for mass-production.